This is a pretty incredible card. Over 9 campaigns, you start to notice that the game often presents specific configurations of scenario mechanics that can all get bodied by a boot in the door:
- A scenario climax with one or two key locations, guarded by scary enemies or effects, that can be cleared by emptying out a high-shroud room (sometimes with a narrative shift or bonus for doing so instead of killing the enemies).
- Optional high-shroud challenge rooms with Victory 1 or Victory 2 attached.
- Rooms with incredibly high shroud values you can only hope to investigate by either following specific mechanics or making specific sacrifices (horror, card discards, clue expenditures).
- Needing to spend many clues for a certain effect, with a room that infinitely replenishes clues.
- Enemies or card effects tied to the shroud value of a room.
- New, in The Scarlet Keys: performing non- tests against a room's shroud value.
This is up there with First Watch in the vein of 'great Guardian utility cards your Fighter will struggle to squeeze 2-4 of into their deck'. And it's a level 0 event, so even the Tony brand of deck building can run it. You might think you specifically need high base to make it work, but almost any teammate wouldn't mind throwing one of their UCs in to the test. A good Mystic will reserve a Promise just for this; a good Rogue will wonder why Skeleton Key even exists.